Thermoplastic vulcanizates (TPVs), wherein an elastomeric component is first dispersed in a thermoplastic resin and then dynamically vulcanized therein, are well known in the art. These compositions, which can be rigid plastics or elastomers depending upon concentration of the rubber phase, retain their thermoplastic nature and can be re-processed at elevated temperatures. This re-processability is a major advantage over chemically crosslinked rubbers and resins since it allows recycling of fabricated parts and results in a considerable reduction of scrap. Such systems often exhibit improved oil and solvent resistance and superior mechanical properties relative to corresponding simple blends wherein the elastomer is not cured. Over the last decade, a variety of TPVs has been disclosed wherein a silicone component is dispersed in an organic resin, such a system being referred to herein as a thermoplastic silicone vulcanizate (TPSiV).
For example, Arkles, in U.S. Pat. No. 4,500,688, discloses semi-interpenetrating networks (semi-IPNs) wherein a vinyl-containing, silicone fluid having, a viscosity of 500 to 100,000 cS is dispersed in a conventional thermoplastic resin. Arkles only illustrates these semi-IPNs at relatively low levels of silicone. The vinyl-containing, silicone is vulcanized in the thermoplastic during melt mixing according to a chain extension or crosslinking, mechanism which employs a silicon hydride-containing, silicone component. Typical thermoplastics mentioned include polyamides, polyurethanes, styrenics, polyacetals and polycarbonates. This disclosure is expanded by Arkles in U.S. Pat. No. 4,714,739 to include the use of hybrid silicones which contain unsaturated groups and are prepared by reacting a hydride-containing, silicone with an organic polymer having unsaturated functionality. Although Arkles discloses a silicone fluid content ranging from 1 to 40 weight percent (1 to 60% in the case of the '739 patent), there is no suggestion of any criticality as to these proportions. Further, although these disclosures illustrate the use of fillers in some compositions, there is no suggestion that such inclusion is critical to any particular property.
Crosby et al. in U.S. Pat. No. 4,695,602 teach composites wherein silicone semi-IPNs vulcanized via a hydrosilation reaction are dispersed in fiber-reinforced thermoplastic resins having, a high flexural modulus (&gt;90,000 psi). The silicones employed are of the type taught by Arkles, cited supra, and the composites are said to exhibit improved shrinkage and warpage characteristics relative to systems which omit the IPN. Although various fillers such as mica, talc or clay are optionally included in the compositions, there is no suggestion that a particular filler or its content is critical to any desired result.
Ward et al., in U.S. Pat. No. 4,831,071, disclose a method for improving the melt integrity and strength of a high modulus thermoplastic resin to provide smooth-surfaced, high tolerance profiles when the modified resin is melt-drawn. As in the case of the disclosures to Arkles et al., cited supra, a silicone mixture is cured via a hydrosilation reaction after being dispersed in the resin to form a semi-IPN, after which the resulting composition is extruded and melt-drawn. This patent specifically illustrates the method for a poly(butylene terephthalate) (PBT) resin wherein 2 weight percent of the silicone IPN is used, although the nature of the silicone components is unclear.
Certain of the above described TPSiVs have also been found to exhibit improved impact resistance relative to unmodified polymers. For example, U.S. Pat. No. 5,648,426, Zolotnitsky describes a method for improving the impact resistance of thermoplastics wherein a mixture of silicone fluids curable by a hydrosilation reaction is dispersed in a first thermoplastic resin and the resulting blend is pelletized. In a second step, a hydrosilation catalyst is sprayed onto the pellets, which are then mixed with a second identical or different thermoplastic resin in a third step. The catalyst is activated in this third step to cure the silicone components and form a semi-interpenetrating network. Zolotnitsky states that any thermoplastic resin can be modified according to this method but only illustrates the use of polypropylene. Again, various fillers are mentioned as optional ingredients, but one of ordinary skill in the art is not directed to any particular system or concentration.
A copending application to Gornowicz et al. entitled "Thermoplastic Silicone Elastomers" (Ser. No. 09/034,089), also teaches the preparation of TPSiVs wherein silicone gum is dispersed in an organic resin and subsequently dynamically vulcanized therein via a hydrosilation cure system. Under certain conditions, systems based on polyolefin or poly(butylene terephthalate) (PBT) resins were shown to have significantly improved mechanical properties over the corresponding simple blends of resin and silicone gum in which the gum was not cured.
Although the above mentioned patents generally disclose thermoplastic silicone vulcanizate compositions, neither these references, nor any art known to applicants, teach the specific components and quantities thereof required to obtain silicone-modified thermoplastic resin systems having exceptional impact resistance, particularly PBT resins.